Magnetism and electromagnetism revision Flashcards
unit for power
watt
what do magnets repel and attract
repel and attract other magnets (depending on poles)
attract magnetic substances
properties of magnetically hard materials
they’re resistant to demagnetising (become permanent magnets)
properties of magnetically soft materials
they make an induced/ temporary magnets
Rules of magnetic fields (+ lines)
-magnetic field lines represent a magnetic field
-always start at north pole and finish at south
-closer together lines are, stronger field is
-field lines never cross
what’s a uniform magnetic field
north and south pole of diff magnets create a field where lines are evenly spaced and all going in smae direction
electric current in a conductor…
… produces a magnetic field around it
describe construction of an electromagnet
wrap coil (aka form a solenoid) around the magnetically soft material (e.g. iron). When the current is switched on, forced domains to line up and magnetizes it. When current is switched off, no longer magnetized.
what happens to a charged particle (or current carrying wire) when moved in a magnetic field.
The wire/current carrier ur using experiences a force, as long as its motion isn’t parallel to the field. This is because of the two interacting fields - one from the wire itself and one from magnetic field it’s been placed in (e.g between two magnets)
why does DC motor work
according to flemings left hand rule, when we reverse the current, we must also flip the direction of force (up to down vice versa). this ensures the motor keeps spinning and doesn’t just wobble
purpose of a spit ring commutator
to reverse direction the current, used in DC motors
how does a loudspeaker work
- current in the electrical current is varying and passes through the coil causing the coil to experience a force (inwards or outwards)
- The current is reversed repeatedly to reverse the force
- The greater the current the greater the force
- As the current and force vary, the coil the vibrates
- The coil is attached to a cone which also vibrates and causes the air molecules to vibrate
- This movement of the air molecules produces the pressure variations in the air needed for a sound wave producing compressions and rarefactions
how can we increase speed of a dc motor (force of a wire)
increase current
increase magnetic field strength
when is voltage induced in a conductor or a coil
when it moves thro a magnetic field or when a magnetic field changes thro it
factors which increase size of induced voltage
speed of wire/coil/magnet
No_ turns on coils of wire
size of coils
strength of magnetic field
how can electricity be generated by a magnet
by moving a magnet inside a coil of wire. the electricity is generated by the coil ‘cutting’ magnetic field lines
what is a basic transformer made up of
a primary coil
a secondary coil
an iron core
how does a transformer work
- An alternating current is supplied to the primary coil
- The current is continually changing direction meaning it will produce a changing magnetic field around the primary coil
- The iron core is easily magnetised, so the changing magnetic field passes through it and there is now a changing magnetic field inside the secondary coil
- This changing field cuts through the secondary coil and induces an alternating potential difference
how can u change the size of an alternating voltage in a transformer
having different number of turns on the input and output sides
what does each finger represent in flemings left hand rule
thumb - force
first - magnetic field
second - current
purposes of step-up/ step-down transformers
- to increase p.d of electricity before it is transmitted across the national grid
- to lower high voltage electricity used in power lines to the lower voltage used in homes
- used in adapters to lower mains voltage to the lower voltages used by many electronic devices
relationship between voltages and No_ turns for a transformer
input (primary) voltage / output (secondary) voltage = primary turns / secondary turns
if we were to assume transformer was 100% efficient, equation between primary and secondary power
power primary = power secondary
primary voltage x primary current = secondary voltage x secondary current
Vp x Ip = Vs x Is
